Sodium selenite penta-hydrate, Na2SeO3·5H2O.

In the crystal structure of Na2SeO3·5H2O [disodium selen-ate(IV) penta-hydrate], two Se, two selenite O atoms and one water O atom are located on a mirror plane, and one water O atom is located on a twofold rotation axis. The coordination of one Na(+) cation is distorted trigonal bipyramidal, formed by three equatorial H2O ligands and two axial selenite O atoms. The other Na(+) cation has an octa-hedral coordination by six water mol-ecules. The two independent SeO3 groups form almost undistorted trigonal pyramids, with Se-O bond lengths in the range 1.6856 (7)-1.7202 (10) Å and O-Se-O angles in the range 101.98 (3)-103.11 (5)°, and both are μ2-O:O-bonded to a pair of Na(+) cations. Hydrogen bonds involving all water molecules and selenite O atoms consolidate the crystal packing. Although anhydrous Na2SeO3 and Na2TeO3 are isotypic, the title compound is surprisingly not isotypic with Na2TeO3·5H2O. In the tellurite hydrate, all Na(+) cations have an octa-hedral coordination and the TeO3 groups are bonded to Na(+) only via one of their three O atoms.